It has been known in microwave communications systems to simultaneously transmit two signals having polarizations orthogonal to each other or to selectively switch between signals of orthogonal polarization. In order to provide the ability to change polarization of microwave radios driving a common antenna, and to do so for a given radio independently of the other(s), it is known to use respective independent couplers between the radios and the antenna and to connect the independent couplers to the antenna through an ortho-mode transducer.
It has been known to provide a directional coupler capable of operation with two operational modes, propagating simultaneously or alternatively, such as described in Kurtz, U.S. Pat. No. 2,817,063, entitled “Balanced Slot Directional Coupler.”
In general, microwave couplers comprise coupled transmission lines. Telecommunications systems widely use waveguide, micro-strip, strip-line and coaxial couplers. One example of a microwave coupler comprises a first elongated waveguide section that is, for example, rectangular or circular in cross-section (transverse to the propagating direction of the electromagnetic wave) and that extends longitudinally in the wave's propagation direction, and a second elongated waveguide section that is also rectangular or circular in cross-section. Assuming a rectangular configuration, the rectangular cross-sectional dimensions of the two waveguide sections may be the same, and the sections are parallel with and adjacent to each other so that they share a common wall. The wall usually defines a single elongated through-slot aligned on the wall's longitudinal center line or a plurality of through-slots that are usually aligned on the wall's longitudinal center line and spaced apart about a quarter wavelength of the electromagnetic wave the coupler propagates. An electromagnetic wave in one of the waveguide sections excites the slots and thereby excites a corresponding electromagnetic wave in the other waveguide section. Such couplers are single-mode couplers when the rectangular cross-section, as is usually the case, does not support orthogonal propagation modes. At sufficiently high frequencies, however, a rectangular waveguide can support two modes. If the polarizations of the two modes are orthogonal to each other, the waveguide could be considered a dual-mode waveguide in such use.
The waveguide sections comprising the coupler can be modified, preferably to a square cross-section or to a rectangular cross-section with appropriate dimensions, as should be understood in this art, so that each section is capable of supporting orthogonal modes. If the slot configuration is also modified so that the row of slots (or elongated single slot) is offset from the center wall's longitudinal center line, and a parallel row of slots (or single slot) is added, for example where the two rows (or two single slots) are disposed symmetrically with respect to the center line, the slots can excite both orthogonal modes from one waveguide section so that both modes propagate in the other waveguide section, as described in Kurtz, U.S. Pat. No. 2,817,063. Because each of the two orthogonal modes in the first waveguide section couples to the same orthogonal mode in the second waveguide section, the first waveguide can simultaneously transmit both orthogonal modes and simultaneously couple both modes to the other waveguide without creating an interfering electromagnetic wave. In this sense, the coupler may be said to electrically isolate the two modes.
In microwave line-of-sight communication links, it is known to connect a single antenna, for example a reflector-type antenna, to a first radio unit and a second radio unit, so that either radio, or both simultaneously, may be used with the same antenna. In some such applications, the second radio is a back-up to the first radio, so that the second radio starts transmitting when the first radio fails to maintain radio communication, until the first radio is replaced. The two radios are connected to the antenna by a single mode coupler, as described above, in which: (a) the first radio is coupled to one end of the first waveguide section, (b) the second radio is coupled to one end of the second waveguide section, on the same end of the coupler as the first radio, (c) the antenna is coupled to the opposing end of the first waveguide section, and (d) the opposing end of the second waveguide section is terminated by a microwave-absorbing element to prevent undesirable microwave reflections Impedance matching is provided at the coupler ports at each radio and at the antenna, as should be understood by those skilled in this art. It is known to have radios operating at different polarizations connect to the same antenna via respective single mode couplers, where the single mode couplers connect to the antenna through an ortho-mode transducer.
It is also known to couple more than two radios to the same antenna, also using single mode coupling.